It is known to form new well paths, sidetracks, from an existing well by drilling a new well path out through the wall of a casing. This is carried out by anchoring a whipstock at a desired location in the well, whereby a drilling body, which is brought into engagement with the whipstock, changes direction and drills from the inside and through the sidewall of the casing. Oftentimes, such a drilling operation leaves an oblong opening, a window, in the wall of the casing. It is customary to form such a sidetrack at a relatively shallow level in a well, and far away from the reservoir, so as to ensure that the casing, which is to be drilled through, is cemented to the surrounding formation. By so doing, the window becomes sufficiently stable, thereby ensuring that there is no danger of the portion having the window being allowed to rotate during subsequent operations, for example when the drill string is pulled back through the window. Given that the sidetrack is formed at a relatively shallow level in the well, the new well path oftentimes becomes very long, which incurs large costs to an operator.